Cation Radical Diels-Alder Reaction of 1,3-Butadiene: A Two-Step Cycloaddition

The structure of the ion-molecule adduct produced in the reaction of 1,3-butadiene cation radical with neutral 1,3-butdiene has been investigated by collisionally stabilizing the adduct and then acquiring its collision-activated decomposition (CAD) spectrum by using a tandem mass spectrometer. The CAD spectrum of the adduct changed dramatically depending on the rate of the stabilization. This observation has been interpreted in terms of two distinct structures for the adduct. One which results at a total pressure of 13 mtorr has been identified as 4-vinylcyclohexene cation radical. This identification is based on the similarity of the CAD spectrum of 4-vinylcyclohexene and of the adduct. The second structure, stabilized at higher pressures (≲130 mtorr), is a branched, acyclic C₈H₁₂ cation radical. The rate constant for the reaction has also been determined. The results of these experiments have been interpreted in terms of a stepwise mechanism for the formation of the 4-vinylcyclohexene cation radical instead of a concerted [4 + 1] cycloaddition mechanism.